Non-traditional high temperature materials, such as ceramic matrix composite (CMC) materials, are more commonly being used for various components within gas turbine engines. As CMC materials can withstand relatively extreme temperatures and pressures, there is particular interest in replacing components formed of traditional materials within the flow path of gas turbine engines with CMC materials. CMC materials, particularly continuous fiber ceramic composite (CFCC) materials, are currently being utilized for shrouds, combustor liners, nozzles, and other high-temperature components of gas turbine engines. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material.
CMC materials generally comprise a fibrous or filamentary reinforcement material embedded within a ceramic matrix material. The reinforcement material serves as the load-bearing constituent, while the ceramic matrix protects the reinforcement material, maintains fiber orientation, and serves to dissipate loads to the reinforcement material. CMC articles are frequently fabricated from multiple layers of “prepreg” or “pregreg tapes” that are typically tape-like structures that include a reinforcement material impregnated with a slurry that contains a precursor and/or powder of the matrix material and one or more organic binders. The prepreg tape undergoes processing (including firing) to convert the precursor or powder to the desired ceramic. Prepregs for CFCC materials frequently include a two-dimensional fiber array comprising a single layer of unidirectionally aligned tows (bundles of individual fiber filaments) impregnated with a matrix precursor or powder to create a generally two-dimensional laminate. Multiple plies of the resulting prepregs are then stacked and debulked to form a laminate preform, a process referred to as “layup.” The prepregs are typically but not necessarily arranged so that tows of adjacent prepregs are oriented transverse (e.g., perpendicular) to each other, providing greater strength in the laminar plane of the preform (corresponding to the principal load bearing directions of the final CMC component).
Following layup, the laminate preform typically undergoes debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave. In the case of melt-infiltrated (MI) CMC articles, the debulked and cured preform undergoes additional processing. First, the preform is heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a carbon char, and produces a porous preform for melt infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the preform is melt infiltrated, such as with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon constituent of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC component.
Conventional methods for making MI-CMC prepreg tapes have included the use of a wet drum winding technique. Typically, a wet drum winding processes entails pulling a fiber tow through a bath containing a slurry mixture that includes suitable matrix precursor or powder materials, organic binders, and solvents, and then winding the resulting wet, precursor/powder-impregnated tow around a drum. Before contacting the drum, the wet, precursor/powder-impregnated tow is preferably pulled through an orifice to control the amount of slurry picked up by the tow in the slurry bath. By indexing the drum (and/or the bath and orifice), the tow is laid down at a constant pitch so that each tow winding touches but does not completely overlap the tow winding from the previous drum revolution, yielding a continuous, unidirectional prepreg tape. Prior to being wound with the tow, the drum is preferably wrapped with a release sheet or carrier sheet, such as a film formed of TEFLON® (polytetrafluoroethylene, or PTFE), so that the resulting prepreg tape can be more easily removed from the drum. The release sheet also acts as a carrier to support the prepreg tape during subsequent handling and cutting. While on the drum, the prepreg tape is typically allowed to air dry by allowing the solvents to evaporate. Alternatively, the tape may be cut from the drum, laid flat, and allowed to air dry.
Prepreg tapes produced by such conventional wet drum winding processes typically have a surface roughness, or waviness, corresponding to the pitch of the fiber tow on the drum. There is also typically variability in the distribution of fiber and matrix across the tape because of the tow pitch. Furthermore, because the tow is under tension during the winding process, the tow tends to be pulled down onto the drum surface, yielding a prepreg tape that has proportionally more tow at the surface of the tape contacting the drum and proportionally more matrix precursor or powder at the surface of the tape facing away from the drum.
Furthermore, prepreg tapes made by such conventional wet drum winding processes can also suffer from a significant amount of broken tow fibers and loosely adhering fibers (i.e., “fuzz”) that can break off and cause blockage of the orifice. When blockage occurs, the amount of slurry remaining on the tow downstream of the orifice is diminished, leading to a region on the resulting prepreg tape with lower than optimum matrix content. In severe cases, the blockage of the orifice can continue to accumulate broken fibers from the tow until it eventually causes the tow to break. In order to prevent such problems from blockage, the orifice is typically sufficiently sized to allow a majority of tow, even those with moderate amounts of damaged fiber and adhering loose fiber, to readily pass. Consequently, the amount of matrix picked up by the tow may be higher than would be optimum, thus leading to lower than desired fiber volume fraction in the finished composite. Even with the use of a large orifice, orifice blockage can still occur. Thus, drum winding operations require constant operator supervision so that such blockages can be removed as they occur.
Another complication of conventional drum winding processes is that the tow must be completely impregnated (i.e., wet out) with slurry during the winding process, which requires that the tow spend a sufficient amount of time submersed in the slurry to allow for complete wet out. This submersion time, which can be about five seconds for certain processes, places a limit on the speed with which the tow can be drawn through the slurry bath.
Other known methods for forming CMC prepreg tapes include dry winding processes. In such processes, a release sheet having an adhesive film is first applied to a drum. Then, a dry fiber tow (i.e., a fiber tow that has not been pulled through a bath containing a slurry mixture) is wound on the drum so that the fibers of the tow adhere to the adhesive film of the release sheet to form a filamentary mat. Then, a slurry mixture that includes suitable matrix precursor or powder materials, organic binders, and solvents is casted into the filamentary mat to yield a prepreg tape. Despite the benefits of removing some of the steps of conventional wet drum processes, such dry winding techniques similarly present a number of challenges. For instance, casting the matrix material onto the filamentary mat while the mat is still on the drum is particularly challenging and results in less than optimal impregnation. Removing the filamentary mat is also undesirable as handling of the filamentary mat is less than ideal, as the fiber filaments are prone to move about as they are only bound with the adhesive film of the release layer. Moreover, when casting the slurry mixture into the filamentary mat, the force to push or engulf the filaments with the slurry matrix is generally insufficient. As such, many times the slurry matrix is not dispersed between all of the filaments of the tow, leading to less than optimal mechanical properties of the final composite article.
Accordingly, improved systems and methods that address one or more of the challenges noted above would be useful.